1. Field of the Invention
The present invention relates to an image forming system for forming an image under the control of a control means having a program memory.
2. Description of the Prior Art
Significant recent developments have been made in electronic engineering. In particular, higher integration of electric or electronic circuits has been achieved, with an end result of a micro computer for controlling various types of equipment. With such a trend, a copy apparatus is increasingly often controlled by a micro computer. In an image forming apparatus such as a copy apparatus, if high performance, high speed processing and multifunctions are required, a plurality of micro computers rather than a single micro computer are required to separately control functions of a copy apparatus main body, an ADF (Auto Document Feeder), a sorter and the like. When a plurality of micro computers are operated to control different functions, means for performing parallel and serial transfer of data between I/O ports of the micro computers have been proposed. However, when a micro computer error occurs during such data transfer, for example, a defective contact error or noise between an interface and main body or between a peripheral and the interface when a peripheral is in use, the cause of such an error is hard to find, especially in the case of serial transfer, so that necessary measures cannot be implemented quickly.
When a serial transfer error is caused by a defective connector contact or by noise during the control of various apparatuses by a serial transfer means, either the copy apparatus main body or a peripheral may operate erratically, leading to a major accident. In a system having two micro computers A and B, for example, when the micro computers exchange a signal REQ.multidot.O signalling a need for data transfer and a signal REQ.multidot.E signalling that data can be received, a serial transfer operation is performed. The micro computer A generates signals REQ.multidot.O at a constant period and awaits signals REQ.multidot.E in return. However, if, for some reason, a signal REQ.multidot.E is not received by the micro computer A due to erratic operation of the program of the micro computer B, noise, a defective interface and the like, the micro computer A continues to monitor the signal REQ.multidot.E and holds up the program. Accordingly, the next processing sequence cannot be commenced.
When a timer is operated to check the signal, a timer is started as the micro computer A generates a signal REQ.multidot.O. It is checked if a signal REQ.multidot.E is received before a predetermined period of time preset in the timer has elapsed to perform diagnosis of the serial transfer means. Meanwhile, at the side of the micro computer B, after the signal REQ.multidot.E is generated, a timer is started by a similar means to that used in the micro computer A. If serial data is not received within the period of time preset in this timer, the micro computer B decides that there has been some abnormality. In such a case, if a data transfer abnormality is generated during a copy process, the respective micro computers may latch in respect of their programs. In particular, when an abnormality is generated during paper feed or exposure lamp lighting in a sequence control micro computer, the state in which the abnormality is generated is latched, and may significantly damage the overall system.